Hi Suz,

What I have read is that each monarch caterpillar will consume 20 or more large leaves. Hope that helps you!

Those kits usually contain enough food for four or so mature caterpillars. (More commonly, you'll find Painted Lady kits.)
The idea is to provide for returning ELFs and future generations.
Have plenty of native Milkweed planted on hand. Be wary of purchasing Milkweed in Big Box stores and nurseries that may use pesticides.
You're better off driving out to the country and collecting your own Milkweed. You may even find some caterpillars to raise.
Also, have plenty of nectar plants on hand that correspond with their migration, such as Mexican Sunflower (Tithonia)